1. Field of the Invention
This invention relates to a system and method for managing a motor vehicle's electric power and more particularly to a system and method for monitoring and determining battery and engine conditions and for controlling an alternator to selectively charge the battery to optimize battery health and performance while efficiently utilizing engine power to improve fuel economy.
2. Description of Related Art
A conventional electric power management system in a motor vehicle such as an automobile uses a battery to supply the vehicle's electrical needs with an alternator or generator (hereinafter "alternator") for charging the battery under regulation by a regulator. Power drawn from the battery is replenished by charging from the alternator, which in turn derives energy from the vehicle's engine. The regulator monitors the battery voltage and regulates the battery voltage by passing or limiting the charge supplied from the alternator. Typically, whenever an engine is operating, the alternator is operated to provide electrical energy to the vehicle and to charge the battery, regardless of battery conditions. The battery provides all electric energy needed by the vehicle when the alternator is turned off. A handbook published by Robert Bosch GmbH and the Society of Automotive Engineers, Inc., "Automotive Electric/Electronic Systems", 1988, pages 18 to 69, describes the principles of operation of alternators and regulators.
Numerous factors contribute to a battery's performance and health. By monitoring simply the battery voltage, as in most conventional automotive battery charge systems, the true condition and health of a battery cannot be determined and battery health and performance cannot be optimized. Factors affecting battery performance include the battery's state of charge, capacity, condition of the battery terminals, level and concentration of the electrolyte, load conditions, plate area, temperature, and internal resistance. Most of these parameters vary depending on operating conditions. When the battery is being charged by the alternator/generator, the voltage measured across the battery (battery charging voltage) is made up largely of two factors: (1) the counterelectromotive force (CEMF) and (2) internal voltage drop. The CEMF is the potential which the battery charging voltage must overcome in order to charge the battery. Factors affecting the CEMF include the battery charging rate, temperature, concentration of electrolyte, plate area in contact with the electrolyte and state of charge. The battery internal voltage drop is caused by current flowing through the internal resistance of the battery. The internal resistance, which includes ohmic resistance and polarization effects of a battery, is made up largely of the normal resistance to current flow inherent in the connectors, connector straps, welded connections, plate area in contact with the electrolyte, battery temperature, electrical resistivity of the electrolyte, and other factors including sulfated or discharged plates and the condition of the battery terminals. The internal battery voltage drop is calculated by multiplying the charging rate in amperes and the battery resistance in ohms.
Of the factors discussed above affecting battery performance, the state of charge (SOC) is among the most important. SOC proportionally affects the CEMF, e.g., if SOC is high, the CEMF is higher and a larger charging voltage is required to further raise the SOC.
U.S. Pat. No. 5,281,919 to Palanisamy (the '919 patent) explains in detail the factors affecting battery operation and discloses an effective system for monitoring and determining battery operating parameters. The '919 patent discloses software for optimizing battery performance and diagnostic routines for fault identifications. U.S. Pat. No. 4,978,942 (the '942 patent), also to Palanisamy discloses an effective technique for dynamically determining the battery internal resistance. The disclosures in both the '919 and '942 patents are incorporated herein by reference.
With advances in microprocessor technology, vehicular controls employing microprocessors have gained in importance and popularity and since battery operation parameters are largely dynamic, it follows that a processor is well suited for monitoring the various battery conditions in place of the conventional regulator. For example, U.S. Pat. No. 5,404,106 to Matsuda et al. describes a system which monitors battery capacity and calculates battery internal resistance based on voltage, current and temperature measurements. U.S. Pat. No. 5,280,232 to Kohl proposes a processor-based device which determines and calculates state of charge by measurements in separate time increments. U.S. Pat. No. 5,193,067 to Sato proposes a similar approach by using a processor-controlled measurement of the battery's electrolyte specific gravity and temperature. Whether a conventional or an `intelligent` regulator/alternator is used, the object of the prior art is to charge the battery to 100% state of charge.
As is well known, the electrical energy produced by the alternator is derived from power or energy drawn from the engine, usually by means of the belt and pulley driven by the engine to rotate the alternator. Systems have been proposed to monitor or control the loading effects of the alternator on the engine. For example, U.S. Pat. No. 5,256,959 to Nagano proposes controlling engine load by controlling the alternator's field current. U.S. Pat. No. 4,789,817 to Asakura proposes minimizing alternator load on the engine when load constraints cause loss of engine speed, and U.S. Pat. No. 4,659,977 to Kissel proposes a microprocessor-based regulator for controlling the charging of the battery. The regulator disclosed in Kissel monitors either local ambient or battery temperature and battery voltage level. Vehicle speed is also measured and this data and the battery data are compared with preset values along with engine RPM to control the field windings of the alternator.
It is also well known that when a vehicle is driven by an engine, a fair amount of the energy produced by the engine is dissipated, and thus wasted, when the driver needs to slow or stop the vehicle. Further, when a vehicle is coasting or cruising at fairly constant speed or traveling downhill, the power drawn from the engine is reduced because the vehicle momentum becomes a large portion of the energy required to maintain the vehicle at speed. For purposes of the present invention, these engine and vehicle operation conditions will be referred herein as "surplus energy" modes. It can be seen that if the battery-alternator-engine system can be managed such that the alternator is turned on to charge the battery when the vehicle and/or engine is operating in the surplus energy mode, the surplus kinetic or potential energy which would otherwise have been wasted is recovered in the form of stored electrical energy, resulting in more energy efficient system usage and thus a reduction in the engine's fuel consumption.
Given the availability of the above proposed systems, there exists a need for an alternator/battery power management system which is capable of monitoring and determining battery, engine, and vehicle operation conditions while controlling alternator operations to minimize the effective power drawn from the engine while optimizing battery operations.